1. Field of the Invention
The present invention relates to a so-called alignment mark or position-alignment mark, and to a laser trimmer widely used in semiconductor device fabrication processes and a semiconductor manufacturing process.
2. Description of the Prior Art
In general, in the semiconductor-device fabrication process, a precise position alignment or so-called "mask alignment" is required each time a mask pattern is reduced in size and transferred to a silicon wafer by optical reduction-photolithographic techniques. A precise position alignment is also required where a laser trimming apparatus cuts a laser fuse on a device pattern.
For example, where the laser trimming apparatus is used to cut the laser fuse of the device pattern, expansion and contraction of the device pattern occurring in the fabrication process often produce differences in the coordinate systems of the device pattern and the laser trimming apparatus. Unless the coordinate system of the device pattern on the silicon wafer (hereinafter referred to as the "design-basis coordinate system") coincides with that of the laser trimming apparatus (hereinafter referred to as the "apparatus-basis coordinate system"), it is impossible for the laser trimming apparatus to project its laser beam light onto a precise position on the wafer. This is because the position of a fuse (to be cut) based on the design data varies during the fabrication process. This variation results in a cutting failure.
Heretofore, a plurality of conventional position-alignment marks have been formed on the device pattern to make the apparatus-basis coordinate system precisely coincide with the design-basis coordinate system of the device pattern. Each of the conventional position-alignment marks is constructed of a highly reflective aluminum (Al), usually of an L- or I-shaped convex planar form. A field region with no opaque layer surrounds each of the position-alignment marks, thus forming a background to it.
The device pattern is scanned with the laser beam along the x-axis or along the y-axis of the design-basis coordinate system. Variation in the amount of the laser beam light reflected vertically from the aluminum layer is thus detected each time it scans a position-alignment mark and its surrounding background. The position of each of the position-alignment marks is thus detected. Then, based on the detected coordinates of the position-alignment marks, the device pattern's position is adjusted so that the apparatus-basis coordinate system precisely coincides with the design-basis coordinate system of the device pattern.
In general, the aluminum layers of the position-alignment marks, as well as the field region surrounding them, are covered with a film such as phosphosilicate glass or plasma nitride. Thus, since the aluminum layers and the field region or field oxide film are covered with a film, the laser beam light is also reflected vertically by the film. Since the laser beam light is coherent, the vertically reflected laser beam lights from the aluminum layer and from its cover film, as well as those from the field oxide film and from its cover film, interfere with each other. Consequently, the vertical reflectance ratios of each of the aluminum layers and the field region or field oxide film depends on the thickness of the cover film from which the laser beam light is reflected. With a certain thickness of cover film, the reflectance ratio of the aluminum layer may be substantially identical to that of the film oxide layer so that the contrast between each of the position-alignment marks and its background is considerably weakened, thus making it impossible to perform a precise position alignment of the device pattern.
To eliminate this possibility, in the conventional method, the cover film deposited on the aluminum film is removed by etching. However, the conventional method is disadvantageous in that each of the position-alignment marks is restricted to a predetermined position some 100 .mu.m away from active region within the device pattern.